Transformer coil construction



Aug. 2, 1960 L, C, PQLZELLA ET AL 2,947,959

TRANSFORMER COIL CONSTRUCTION Filed Nov. 28, 1955 INVENTORS 0L/S C. PoZze ZZa/ WL m A Wok/WM5' 'United States Patent 0 ,'leA

TRANSFORMER COIL CONSTRUCTION Louis C. Polzella, Arthur E. Snowdon, and William P. Carpenter, Bristol, Conn., assignors to The Superlor Electric Co., Bristol, Conn., a corporation of Connecticut Filed Nov. 28, 1955, Ser. No. 549,295

1 Claim. (Cl. 336-198) i This invention relates to transformer construction and more particularly to a protective means for iron cores used in transformers.

ln transformers having an iron core, it has long been realized that a protective means for the core shouldbe employed to protect the core from eventual ruin by the corrosive action of the moisture in the air on the iron. In addition thereto, certain manufacturing operations performed when constructing the core accelerate the corrosion. VFor example, where the windings of the transformer are to be plated, such as with rhodium, the only feasible method of plating requires that a portion of the iron core be submerged in the rhodium bath. Even with great care in washing the transformer after the dipping, a portion of the rhodium solution remains on the core which upon drying out form salts thereon. When moisture from the air contacts the core, it combines with the salts to form an acid solution which reacts with the iron, causing an increased corroding or rusting of the iron core. Moreover,Y a reaction between the iron and plating solution would contaminate the solution.

An object of the present invention is to provide a protective covering for surfaces of a core ofa transformer which'forms a protective barrier for the surfaces so covered.

Heretofore, protective casings for a transformer core have been costly, requiring special machinery and excessive labor, with the result wherever practical, that such casings have not been used. However, one method of forming a protective layer on the core consists of spraying the core with a varnish or other liquid sheet forming material and then baking the core to harden the material. This required a spraying booth with the usual loss of spray material and also the extra steps of spraying and baking. Moreover, a varnish deposited in such a manner does not form a complete barrier against corrosive elements since pin holes or minute openings are present. Another method that may be employed, utilizes a tape of the type which may be adhesive and waterproof or which may be just adhesive and then sprayed with moistureproof solution and baked as in the prior method. ltwill be apparent -that such methods in addition to being expensive, require special apparatus and machinery plus being time-consuming.

Another object of this invention is to provide a protective casing for surfaces ofy a core of a transformer which is economical vto manufacture, facilitates the assembly of the-transformer and inV which there are no special machinery or time-consuming steps required in the Y to the adjacent surfaces.

2,947,959 Y Patented Aug. 2, 196i) amanner has surfaces which are of the shape of the sheets or laminae and surfaces formed by the edges of the laminations; hereafter, the latter are referred to as the laminar surfaces of the core. The laminar surfaces are more susceptible to corrosion because there are crevices formedrbetween the layers which permits a greater area of iron to be subject to corrosive action per unit of surface area and also these crevices may act as capillary tubes causing corrosive elements to enter further into the core. Moreover, corrosive action in the laminar surfaces causes a greater distortion of the core than on the other surfaces with, accordingly, a quicker ruination of the transformer.

A further object of this invention is to provide a protective means for the laminar surfaces of a core which not only covers the surfaces but also penetrates the crevices between the laminae to form an effective barrier against corrosive elements.

The broader aspects of the invention are applicable to either a rectangular or annular core. However, in the specific embodiment of the invention illustrated and described, the protective means is shown in conjunction with and applied to an annular core of the coil of a variable autotransformer. In manufacturing such a coil, a length of thin sheet iron is spirally wound to form the core, a tube of stiffened protective material such as` Bakelite may be inserted in the center and an outer peripheral wrapping which may be of coated paper, varnishe'd cambric or glass tape may be applied to the periphery ofthe core. The core is Athen provided with two end caps which may be also Bakelite or other hard plastic'or insulating material, anda length of electrically conducting wire is toroidally wrapped about this unit. The assembly is then coated with varnish or other protective'material and baked. The'next operation consists of milling or grinding the portions of the convolutions adjacent one of the end caps to form a flat commutator surface on which a brush rides, and finally the commutator surface is dipped into a rhodium bath solution to plate the surface.

One feature of the present invention resides in the provision of having protective means insertable between the side surfaces, which are also the laminar surfaces of the core, and the end caps. This protective means in the illustrated embodiment is shaped to t the laminar surfaces and accordingly are annular. They advantageously consist of a somewhat stiff material such as paper, having adhered on one surface only a heat-activated dry film composed of a synthetic thermosetting resin. The film softens and flows when heat is applied, and bonds itself One example of such a film is known as Permacel 18. When the film softens, it flows into the crevices between the laminaeforming the core and .also flows to seal the crack `between the peripheral covering and the annular protective member and the crack between the annular protective member, and the tube in the center of the core. Moreover, the heat required to bake the varnish is also the heat required to soften and thermoset thelm. After the film has been thermoset, it is impervious to corrosive elements such as moisture and the manufacturing solutions used in constructing the coil.

Other features and advantages will hereinafter appear. Inthe accompanying drawings:

Figure l is a sectional view through an autotransformer coil.

Fig. 2 is an exploded, partial sectional view showing the core, the protective means and the end caps before assembly.

Fig. 3 is an enlarged and exaggerated view of a laminar surface of the transformer core after heat has been 3 applied to melt the ilm and the lm has set as a coating and impregnant.

Fig. 4 is a view of one of the annular protective members with a portion broken away to show the underlying stiif layer.

In constructing the annular autotransformer coil of the illustrated embodiment, the core, generally indicated by the reference numeral 10, is formed by spirally Winding tightly, a length of thin sheet iron or steel to the desired thickness. Each of the whorls in a cross-section of the core as shown in Figs. 1, 2 and 3 forms one of the layers 11 in between which there are minute crevices 12. Preferably, a tube 13, which may be formed of a hard insulating material such as Bakelite is inserted in the center of the core. The outer periphery of the core in the instant embodiment is covered by a tape 14 having a width substantially equal to the width of the sheet of iron wound thereabout. The tape 14 may be of varnished cambric or paper or woven glass Iibers, but preferably of varnished paper. Annular end caps 15 having a crosssectional shape as illustrated in Figs. 1 and 2 are placed on opposite sides of the core 10, and the protective means generally indicated by the reference numeral 18 is positioned therebetween. An electrical winding 16 consisting of a length of insulating copper wire is toroidally wound about the core and the end caps 15. The winding assumes a shape corresponding to the configurations of the assembled end caps and core. The unit as thus far constructed is then completely coated with a varnish or similar substance and baked to harden the coated material. This material besides forming insu-lation also assists in maintaining the Winding convolutions in position. A commutator surface 17 is milled or ground on the portions of the winding 16 on a flat portion a of the end cap 15 to provide the necessary surface on which the autotransformer brush may ride. The coil is then dipped into a rhodium bath to plate the surface 17 with rhodium to provide a pit resistant surface and also a good wear resistant surface. When dipping the unit into the bath an end cap 15 and a part of the core 10 also enter the solution. Without the protective means 18, some of the solution would enter the crevices 12 and upon drying, form salts which, when later combined with moisture from the atmosphere would produce an acid solution that would attack the iron core causing excessive corrosion.

The protective means, generally indicated by the reference numeral 18, is employed to prevent the entrance of the rhodium solution into the crevices 12 of the core 10 and also to seal the sides of the core 10 from moisture. The protective means 18 is shown in Fig. 4 and consists of an annular disk composed of two layers 19 and 20. The Alayer 20 is approximately .002 thick and is a heatactivated dry lm composed of a synthetic thermosetting resin. While the layer 19 may be of any stil material to maintain the shape of the film, it is preferably formed of paper which has been treated to be substantially nonporous to prevent the lm 20, when soft, from iowing therethrough.

The layer 20 and layer 19 are preferably adhered together to facilitate the handling of both layers together without deformation. The layer 20 shown in Fig. 2 is positioned against the laminar surface or side of the core 10 and conforms substantially to the shape of the surface The outer diameter of the disk 18, in the present embodiment, is equal to or a little larger than the outside diameter of the core 10 and tape 1-4, While the inner diameter of the aperture in the disk 18 is equal to the inside diameter of the tube 13.

During the baking or heating operation the tempera'- ture required to bake the varnish is also the temperature required to soften and thermoset the layer 20. Moreover, in view of the sequence of operations previously set forth it will be apparent that the winding exerts a compressive force on the end caps 15, the disks 18 and the sides of the core which cause the softened lm to more readily flow into cracks and crevices and to conform to the space between the sides of the core and the end caps.

In the detailed enlarged view of Fig. 3 showing a section of the coil after the baking step, portions 21 of the layer 28 have entered into and become bonded to the layers 11, sealing the minute crevices 12. Also the cracks formed between the adjacent surface of the protective means 18 and the tube 13 and tape 14 are sealed by portions 22 of the layer 20 squeezed out by the compressive force exerted thereon. This electuates a tight, moistureproof casing for the transformer core which is impervious to atmospheric moisture and liquid solutions such as the rhodium bath solution and the grinding compounds used in forming the commutator surface 17.

1t will be appreciated that the use of the protective means 18 consisting of the layers 19 and 20 result in a great economy in manufacture and the formation of a cheap, though impervious, protection for surfaces of the core. Furthermore, if need should occur to salvage the core 10, the end caps 15 are not bonded or adhered thereto because the layer 19 is interposed therebetween which provides for disassembling without mutilation of the core o1' end caps. Though the protective means has been shown as being annular, it will be apparent that the shape is dictated by the shape of the surfaces of the transformer to be protected and accordingly, should not be limited solely to an annular disk.

Variations and modifications may be made within the scope of the claim and portions of the improvements may be used Without others.

We claim:

An annular coil for a variable autotransformer comprising an annular core formed of thin sheet iron spirally wound and presenting laminar side surfaces; annular end caps positioned against the laminar side surfaces of the core and a toroidal winding of wire wound about the core and end caps; the improvement which comprises protective means for the core including two annular protective disks, each disk consisting of a thermoset resin lm and a layer of paper, positioned between the end caps and the laminar side surfaces of the core, said lm coating the laminar side surfaces of the core, and having portions penetrating spaces in the laminar'side surfaces of the core to provide a protection for the laminar side surfaces of the core, and said paper adhering to the lm and being substantially non-porous and non-perforate and engaging its associated end cap but being unbonded thereto.

References Cited in the file of this patent UNlTED STATES PATENTS 1,978,568 `Crossley et al. Oct. 30, 1934 2,009,013 Karplus July 23, 1935 2,089,434 Schermerhorn Aug. 10, 1937 `2,494,180 Koubek Ian. 10, 1950 V2,569,468 Gaugler Oct. 2, 1951 2,579,560 Ford Dec. 2S, 1951 2,623,920 Ford Dec. 30, 1952 2,689,396 Vienneau Sept. 21, 1954 2,696,659 McCarty Dec. 14, ,11954 

